Synchronous first-in/first-out block memory for a field programmable gate array

ABSTRACT

The present invention comprises a field programmable gate array that has a plurality of dedicated first-in/first-out memory logic components. The field programmable gate array includes a plurality of synchronous random access memory blocks that are coupled to a plurality of dedicated first-in/first-out memory logic components and a plurality of random access memory clusters that are programmably coupled to the plurality of dedicated first-in/first-out memory logic components and to the plurality of synchronous random access memory blocks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/448,259, filed May 28, 2003 is now a U.S. Pat. No. 6,838,902.

BACKGROUND OF THE SYSTEM

1. Field of the System

The present system relates to field programmable gate array (FPGA)devices. More specifically, the system relates to a synchronous firstin/first out memory module for an FPGA.

2. Background

FPGAs are known in the art. An FPGA comprises any number of logicmodules, an interconnect routing architecture and programmable elementsthat may be programmed to selectively interconnect the logic modules toone another and to define the functions of the logic modules. Toimplement a particular circuit function, the circuit is mapped into thearray and the appropriate programmable elements are programmed toimplement the necessary wiring connections that form the user circuit.

An FPGA core tile may be employed as a stand-alone FPGA, repeated in arectangular array of core tiles, or included with other functions in asystem-on-a-chip (SOC). The core FPGA tile may include an array of logicmodules, and input/output modules. An FPGA circuit may also includeother components such as static random access memory (SRAM) blocks.Horizontal and vertical routing channels provide interconnectionsbetween the various components within an FPGA core tile. Programmableconnections are provided by programmable elements between the routingresources.

An FPGA circuit can be programmed to implement virtually any set ofdigital functions. Input signals are processed by the programmed circuitto produce the desired set of outputs. Such inputs flow from the user'ssystem, through input buffers and through the circuit, and finally backout to the user's system via output buffers. The bonding pad, inputbuffer and output buffer combination is referred to as an input/outputport (I/O). Such buffers provide any or all of the followinginput/output (I/O) functions: voltage gain, current gain, leveltranslation, delay, signal isolation or hysteresis.

As stated above, many FPGA designers incorporate blocks of SRAM intotheir architecture. In some applications, the SRAM blocks are configuredto function as a first-in/first-out (FIFO) memory. A FIFO is basically aSRAM memory with automatic read and write address generation and someadditional control logic. The logic needed to implement a FIFO, inaddition to the SRAM blocks, consists of address generating logic andflag generating logic.

Counters are used for address generation. Two separate counters are usedin this application for independent read and write operations. Bydefinition, a counter circuit produces a deterministic sequence ofunique states. The sequence of states generated by a counter is circularsuch that after the last state has been reached the sequence repeatsstarting at the first state. The circular characteristic of a counter isutilized to generate the SRAM's write and read addresses so that data issequenced as the first data written to the SRAM is the first data read.The size of the sequence produced by the counters is matched to the SRAMaddress space size. Assuming no read operation, when the write countersequence has reached the last count, the SRAM has data written to allits addresses. Without additional control logic, further writeoperations would overwrite existing data starting at the first address.

Additional logic is needed to control the circular sequence of the readand write address counters in order to implement a FIFO. The controllogic enables and disables the counters when appropriate and generatesstatus flags. The read and write counters are initialized to produce acommon start location. The control logic inhibits reading at anylocation until a write operation has been performed. When the writecounter pulls ahead of the read counter by the entire length of theaddress space, the SRAM has data written to all its addresses. Thecontrol logic inhibits overwriting an address until its data has beenread. Once the data has been read, the control permits overwriting atthat address. When the read counter catches up to the write counter, theSRAM no longer contains valid data and the control logic inhibitsreading until a write operation is performed.

Output signals, known to those of ordinary skill in the art as flags,provide the system with status on the SRAM capacity available. The fulland empty conditions are indicated through full and empty flags. Twoadditional flags are generated to warn of approaching empty or fullconditions.

FPGAs have programmable logic to implement this control logic. With theavailability of a SRAM block, an FPGA application may be configured tooperate as a FIFO memory. Many prior art FPGAs use this approach.However, considerable FPGA gates are consumed when implementing thecontrol logic for a FIFO in this manner and this increases the cost ofthe application. Also, the performance of the FIFO is likely to belimited by the speed of the control logic and not the SRAM.

Hence, there is a need for an FPGA that has dedicated logic specificallyincluded to implement a FIFO. The FIFO logic may included among the SRAMcomponents in an FPGA core tile. The result is improved performance anda decrease in silicon area needed to implement the functions withrespect to implementing the FIFO-function with FPGA gates.

SUMMARY OF THE SYSTEM

A field programmable gate array having a plurality of random accessmemory blocks coupled to a plurality of dedicated first-in/first-outmemory logic components and a plurality of random access memory clustersprogrammably coupled to the rest of the FPGA is described.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription of the invention and accompanying drawings which set forthan illustrative embodiment in which the principles of the invention areutilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a one-tile FPGA of the present system.

FIG. 2 is a block diagram of an FPGA including multiple core tiles 102as shown in FIG. 1.

FIG. 3 is a simplified block diagram of a synchronous random accessmemory (SRAM) module of the present system.

FIG. 4 is a simplified schematic diagram illustrating the FIFO logiccomponent of the present system.

FIG. 5 is a simplified block diagram illustrating the architecture of aRAM cluster of the present system.

FIG. 6 is a simplified schematic diagram illustrating RT module, RNmodule, RI module and RO module of a RAM cluster of FIG. 5.

FIG. 7 is a simplified schematic diagram illustrating RC module of a ramcluster of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

In the present disclosure, Vcc is used to define the positive powersupply for the digital circuit as designed. As one of ordinary skill inthe art will readily recognize, the size of a digital circuit may varygreatly depending on a user's particular circuit requirements. Thus, Vccmay change depending on the size of the circuit elements used.

Moreover, in this disclosure, various circuits and logical functions aredescribed. It is to be understood that designations such as “1” and or“0” in these descriptions are arbitrary logical designations. In a firstimplementation of the invention, or “1” may correspond to a voltagehigh, while “0” corresponds to a voltage low or ground, while in asecond implementation, “0” may correspond to a voltage high, while “1”corresponds to a voltage low or ground. Likewise, where signals aredescribed, a “signal” as used in this disclosure may represent theapplication, or pulling “high” of a voltage to a node in a circuit wherethere was low or no voltage before, or it may represent the termination,or the bringing “low” of a voltage to the node, depending on theparticular implementation of the invention.

FIG. 1 is a block diagram of an illustrative core tile 102 in an FPGA100 of the present system. FPGA core tile 102 comprises an array oflogic clusters 104, static random access memory (SRAM) clusters 106 andstatic random access memory (SRAM) modules 108. Logic clusters 104 andSRAM clusters 106 are connected together by a routing interconnectarchitecture (not shown) that may comprise multiple levels of routinginterconnects. FPGA core tile 102 is surrounded by input/output (I/O)clusters 110, input/output (I/O) FIFO control blocks 114 andinput/output banks 112. There are two rows of I/O clusters 110 on thetop and bottom edges of FPGA 100 and one column of I/O clusters on theleft and right edge of FPGA 100. In the present example, forillustrative purposes only, there are seven SRAM clusters 106 adjacentto and interacting with each SRAM module 108.

FIG. 2 is a block diagram of an illustrative FPGA including multiplecore tiles 102 as shown as an example in FIG. 1. As shown in FIG. 2,FPGA 120 comprises four core tiles 102, though other numbers of tilesare possible. Core tiles 102 are surrounded by I/O clusters 110,input/output FIFO control blocks 114 and I/O banks 112.

FIG. 3 is a simplified block diagram of a static random access memory(SRAM) block 108 of the present system. The present system combinesdedicated control logic with a two port SRAM to produce a FIFO. As setforth in FIGS. 1 and 2, there are four SRAM blocks 108 along the oneside of FPGA tile 102. Each SRAM block 108 may be configured to operateas an individual SRAM module or modules may be cascaded together toproduce wider or deeper memory combinations. As set forth in greaterdetail below, dedicated FIFO control logic has been added to each SRAMblock.

Referring still to FIG. 3, SRAM block 108 comprises a SRAM component150. SRAM component 150 is a memory component. Memory components arewell known to those of ordinary skill in the relevant art and can varygreatly depending on the application. Write data bus 152 and writeaddress bus 156 are coupled to SRAM component 150 through register 154.Write data enable signal lines 158 are each coupled to SRAM component150 through one input of two-input XOR gates 160, 162, 164, 166, 168,AND gate 170 and register 154. The second input of two-input XNOR gates160, 162, 164, 166, 168 is provided by write enable control lines 172.Register 154 receives a clock signal through write clock signal line159. Read address bus 174 is coupled to SRAM component through register176. Read enable signal lines 178 are each coupled to SRAM component 150through one input of XOR gates 180, 182, 184, 186, 188, AND gate 190 andregister 176. The second input of register 176 receives a clock signalthrough read clock signal line 192. Input signal busses 194 and 196provide the signals for determining the write word width and read wordwidth respectively. Read data bus 198 is coupled to the output of SRAMcomponent 150 through register 199 and two-input multiplexer 197.

In the present example, for illustrative purposes only, SRAM block 108has multiple bits accessible by two independent ports: a read only port(all circuitry on the right of SRAM block 108) and a write only port(all circuitry on the left of SRAM block 108). Both ports may beindependently configured in multiple words by bits per wordscombinations. For example, both ports may be configured as 4,096×1,2,048×2, 1,024×4, 512×9, 256×18 and 128×36. In additions a plurality ofSRAM blocks may be cascaded together by means of busses 152, 156, 158,174, 178, 198. In the present example, there are five enable lines foreach port, one for real enable and four for higher order address bits.The ten XOR gates are used to invert or not invert the lines on ablock-by-block basis effectively making AND gates 170 and 190 decoderswith programmable bubbles on the inputs. The write port is synchronousto the write clock and the read port is synchronous to the read clock.As one of ordinary skill in the art would readily recognize, the aboveexample is illustrative only, many other configurations or memory blockscould be used.

FIG. 4 is a simplified schematic diagram illustrating the FIFO logiccomponent 200 of the present invention. FIFO logic component 200 iscoupled between static random access memory (SRAM) clusters 106 andstatic random access memory (SRAM) block 108. In the present example,for illustrative purposes only, FIFO logic component 200 is coupledbetween seven static random access memory (SRAM) clusters 106 and staticrandom access memory (SRAM) block 108. Two input AND gate 202 has itsnon-inverting and inverting inputs coupled to random access memorycluster 106 via signal lines 240 and 242 respectively and an outputcoupled to address comparator 238, and to subtractor circuit 222 throughcounter 210 and to address comparator 232 through registers 218 and 220.The output of two-input AND gate 202 may also be coupled to RAM module108 through tri-state buffer 206. The output of counter 210 may also becoupled to SRAM block 108 through tri-state buffer 214. Two input ANDgate 204 has its non-inverting and inverting inputs coupled to SRAMcluster 106 through signal lines 244 and 246 respectively and its outputcoupled to address comparator 232 through counter 212. Two input ANDgate 204 also has its output coupled to address comparator 238 throughcounter 212, register 224 and register 226 and its output is alsocoupled to subtracter 222 through counter 212. The output of two-inputAND gate 204 may also be coupled to SRAM module 108 through tri-statebuffer 208. The output of counter 212 may also be coupled to SRAM block108 through tri-state buffer 216. Buffers 206, 208, 214 and 216 receivetheir control signals from SRAM clusters 106 programmable configurationbits 248.

Referring still to FIG. 4, subtractor circuit 222 has its output coupledto one input of magnitude comparators 234 and 236. Magnitude comparators234 and 236 receive their second input from the programmableconfiguration bits 228 and 230 respectively. The configuration bits in228 and 230 are programmable threshold values need to generate thealmost full and almost empty flags respectively.

Read data bus 250 and write data bus 252 are coupled directly to SRAMblock 108. When the FIFO logic component is not active, controller bits248 are set at 0 disabling the tri-state buffers 206, 208 214 and 216.When the SRAM is not configured as a FIFO, all input signals originatefrom adjacent SRAM clusters 106. When a SRAM is configured as a FIFO, aselect set of signals from the RAM cluster modules are set to highimpedance and FIFO logic component 200 seizes control of the signallines. When FIFO logic component 200 is active, it seizes control of thewrite enable signals 158, the read enable signals 178 and the read andwrite address lines 174 and 156 respectively as shown in FIG. 3.

Counters 210 and 212 are binary counters, however, they also generategray code. Gray code or “single distance code” is an ordering of 2^(n)binary numbers such that only one bit changes between any twoconsecutive elements. The binary value is sent to subtractor 222 tocalculate the difference between the read and write counters for thealmost full and almost empty flags. The gray code is sent to addresscomparators 232 and 238 as well as to tri-state buffers 214 and 216. Ingray code, one and only one bit changes between any two consecutivecodes in the sequence. The purpose of registers 218 and 220 is tosynchronize the read counter address in 210 to write clock signal andthe purpose of registers 224 and 226 is to synchronize the write counteraddress to read clock signal for comparison purposes. Because there isno requirement that read clock signal 253 and write clock signal besynchronous, there is no guarantee that the outputs of 210 will not bechanging during the setup and hold time windows of register 218. Becauseof the likelihood of change during the register setup and hold timewindow, there is a chance of an uncertain result. The chance of anuncertain result is limited by using gray code to make sure that onlyone bit can change at a time. However the uncertainty on that one bitresolves itself, the result is that the bit will either get the lastaddress or the next address and no other address when comparing the readand write addresses.

When the memory is full writing must be inhibited to prevent overwritingvalid data in the SRAM. To control this the comparison between the readand write addresses is done in the write clock (WCK) time domain sincewrite operations are synchronous to WCK. The read address counter 210gray code sampled two WCK cycles in the past by registers 218 and 220 iscompared to the current write address counter 212 gray code bycomparator 232. If the result is equal, then the SRAM may be full andwriting is inhibited. There is no way to reliably know for certain ifthe SRAM is really full. The read address being compared is two WCKcycles old and one or more read operations may have occurred during thattime. However, by erring on the side of safety when it is possible thatthe memory might be full, overwriting of data can be reliably prevented.

In a similar manner, when the memory is empty reading must be inhibitedto prevent outputting invalid data from the SRAM. To control this thecomparison between the write and read addresses is done in the RCK timedomain since read operations are synchronous to RCK. The write addresscounter 212 gray code sampled two RCK cycles in the past by registers224 and 226 is compared to the current read address counter 210 graycode by comparator 238. If the result is equal, then the SRAM may beempty and reading is inhibited. There is no way to reliably know forcertain if the SRAM is really full. The write address being compared istwo RCK cycles old and one or more read operations may have occurredduring that time. However, by erring on the side of safety when it ispossible that the memory might be empty, reading of invalid data can bereliably inhibited.

Since both a full and an empty condition are detected by equalitybetween the read and write addresses, a way to tell the differencebetween the two conditions is require. This is accomplished by having anextra most significant bit (MSB) in counters 210 and 212 which is notpart of the address space sent to the SRAM block (and not shown in FIG.4 to avoid overcomplicating the disclosure and obscuring the invention).Additional logic (also not shown) inside each comparator 232 and 238compares the read and write MSBs. When the two MSBs are equal and theread and write addresses are equal in comparator 238, this implies apossible empty condition. When the two MSBs are not equal and the readand write addresses are equal in comparator 232, this implies a possiblefull condition.

FIG. 5 is a simplified block diagram illustrating the architecture of aRAM cluster 106 of the present system. As would be clear to those ofordinary skill in the art having the benefit of this disclosure, RAMcluster 106 may comprise any number of the logic components as indicatedbelow. The examples set forth below are for illustrative purposes onlyand in no way limit the scope of the present invention. Random accessmemory clusters 106(0–6) further comprise two sub-clusters 300 and 302.Each sub cluster 300 and 302 has two transmitter modules 314 and tworeceiver modules 312. Right sub cluster 302 has a buffer module 316.

To avoid overcomplicating the disclosure and thereby obscuring thepresent invention, receiver modules 312, transmitter modules 314 andbuffer module 316 are not described in detail herein. The implementationof receiver modules 312 and transmitter modules 314 suitable for useaccording to the present system is disclosed in co-pending U.S. patentapplication Ser. No. 10/323,613, filed on Dec. 18, 2002, and herebyincorporated herein by reference. The implementation of buffer modules316 suitable for use according to the present system is disclosed inU.S. Pat. No. 6,727,726, issued Apr. 27, 2004, and hereby incorporatedherein by reference.

In the present example, for illustrative purposes only, the interface toeach SRAM block 108 is logically one RAM cluster 106 wide and seven rowslong. Thus, there is a column of seven RAM clusters 106(0) through106(6) for every SRAM block 108. Sub-clusters 300 and 302 of RAM cluster106(0) each have one RAM clock interface input (RC) module 304, sixsingle ended input (RT) modules 306 and two RAM interface output (RO)modules 308 in addition to the two transmitter modules 314 and tworeceiver modules 312 as set forth above. Right sub cluster 302 also hasa buffer module 316. RC modules 304 in RAM cluster 106(0) select thewrite and read clock signals from all the HCLK and RCLK networks or fromsignals in either of two adjacent two routed channels and determinetheir polarity. RC modules 304 will be discussed in greater detailbelow. Each RT module 306 provides a control signal to SRAM module 108which is either routed from a single channel or tied off to logic 1 orlogic 0. RO modules 308 transmit read-data or FIFO flags from SRAMmodule 108 into an individual output track. RT modules 306 and ROmodules 308 will be discussed in greater detail below.

Sub-clusters 300 and 302 of RAM clusters 106(1–6) each have threetwo-input RAM channel-up/channel-down non-cascadable signal (RN) modules310, three RO modules 308 and six two-input RAM channel-up/channel-downcascadable signal (RI) modules 309 in addition to the two transmittermodules 314 and two receiver modules 312 as set forth above. Right subcluster 302 also has a buffer module 316. RN modules 310 and RI modules309 provide an input signal to SRAM module 108 that can be routed fromtwo rows, the one in which it is located and the row immediately aboveit.

FIG. 6 is a simplified schematic diagram illustrating RT module 306, RNmodule 310, RI module 309 and RO module 308 of a RAM cluster of FIG. 5.RT module 306 comprises a buffer 354 that has an input programmablycoupled to a horizontal routing track in routing architecture row 352.As is known to those of ordinary skill in the art, there are types ofprogrammable elements. Illustrative examples of such programmableelements include, but are not limited to, MOS transistors, flash memorycell and antifuses. Buffer 354 has an output that is coupled to SRAMblock 108.

RN module 310 comprises a two-input AND gate 356 and a buffer 358. Oneinput of two-input AND gate 356 is programmably coupled to a horizontalrouting track in routing architecture row 350. The second input oftwo-input AND gate 356 is programmably coupled to a horizontal routingtrack in routing architecture row 352. The output of two-input AND gate356 is coupled to SRAM module 108 through buffer 358.

RI module 309 comprises a two-input NAND gate 376 having the ability toselect a signal from routing architecture row 150 or 152. Two-input NANDgate 376 has an output coupled to SRAM block 108 through tri-statebuffer 380 and one inverted signal input of a two-input OR gate 378.Two-input OR gate has a second input coupled to Vcc or ground and itsoutput coupled to SRAM module 108 through tri-state buffer 380. In thepresent disclosure, Vcc is used to define the positive power supply forthe digital circuit as designed. As one of ordinary skill in the artwill readily recognize, the size of a digital circuit may vary greatlydepending on a user's particular circuit requirements. Thus, Vcc maychange depending on the size of the circuit elements used.

In this disclosure, various circuits and logical functions aredescribed. It is to be understood that designations such as “1” and “0”in these descriptions are arbitrary logical designations. In a firstimplementation of the invention, “1” may correspond to a voltage high,while “0” corresponds to a voltage low or ground, while in a secondimplementation, “0” may correspond to a voltage high, while or “1”corresponds to a voltage low or ground. Likewise, where signals aredescribed, a “signal” as used in this disclosure may represent theapplication, or pulling “high” of a voltage to a node in a circuit wherethere was low or no voltage before, or it may represent the termination,or the bringing “low” of a voltage to the node, depending on theparticular implementation of the invention.

RO module 308 comprises a buffer 360 having an input coupled to FIFOcontrol block 200 or SRAM block 108. The output of buffer 360 requiresprogramming voltage protection and drives an output track which inrouting architecture row 352.

FIG. 7 is a simplified schematic diagram illustrating RC module 304 of aram cluster of FIG. 5. RC module 304 comprises a four input multiplexer362 having inputs coupled to the clock network bus 370 (not shown).Multiplexer 362 has an output coupled to a first input of a two-inputmultiplexer 365. The second input of two-input multiplexer 365 isselectively programmably coupled to the routing architecture in rows 372and 374 through two-input AND gate 364. Two-input multiplexer 365 has anoutput coupled to an input of a two-input XNOR gate that has a secondinput programmably coupled to Vcc or ground in routing architecture row372. The output of XNOR gate 366 is coupled to SRAM block 108 throughbuffer 368.

While embodiments and applications of this system have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The system, therefore, is not to berestricted except in the spirit of the appended claims.

1. A field programmable gate array having a routing architecture and aplurality of dedicated first-in/first-out memory logic componentscomprising: a plurality of static random access memory blocks coupled toa plurality of dedicated first-in/first-out memory logic components; anda plurality of random access memory clusters coupled to the plurality ofdedicated first-in/first-out memory logic components and to saidplurality of static random access memory blocks and programmably coupledto the field programmable gate array routing architecture.